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Scientific Papers 


Bureau of Standards 

S. W. STRATTON, Director 

No. 402 



HENRY S. RAWDON, Physicist 

Bureau of Standards 

NOVEMBER 12, 1920 


Sold only by the Superintendent of Documents, Government Printing Office 
Washington, D. C. 





By Henry S. Rawdon 



I. Introduction 715 

II. Method of etching 715 

1. Principles underlying the action 715 

2. Method of application 716 

III . Typical results 717 

1 . Crystalline heterogeneity 717 

2 . Chemical unhomogeneity 720 

IV. Discussion and summary 722 


In the metallographic examination of any metallic material a 
study of the macrostructure occupies a prominent and important 
place. It is generally necessary to use special reagents for reveal- 
ing the different structural features and many such etching solu- 
tions have been described and their use is now a routine matter 
in most metallographic laboratories. One of the most widely 
used reagents for revealing the macrostructure of copper alloys, 
particularly the brasses and bronzes, is ammonium persulphate. 
This has also been suggested as a means for revealing the micro- 
structure of iron 1 though not used to any appreciable extent. 
The advantages of its use as a macroscopic etching medium for 
iron and steel appear to have been very largely overlooked. In 
the following discussion the use of ammonium persulphate for 
this purpose and the structural features which it reveals in a par- 
ticularly striking manner are pointed out. 



Ammonium persulphate is a very strong oxidizing agent and 
decomposes readily when dissolved in water. According to 

1 J. Czochralski, Principles of Etching Phenomena and Metallographic Etching Processes; Stahl und 
Eisen; 35, No. II, p. 1073, 1915. 

13509°— 20 715 

7i 6 Scientific Papers of the Bureau of Standards [Voi.i6 

Treadwell, 2 the fundamental reaction which occurs when water is 
added to a persulphate is represented by the following equation: 

(i) 2 (NH 4 ) 2 S 2 8 + 2H 2 = 2 (NH 4 ) 2 S0 4 + 2 H 2 S0 4 + 2 . 

A further reaction occurs with ammonium persulphate by which 
some of the ammonia is oxidized to the form of nitrate. 

(2) 8(NH 4 ) 2 S 2 8 + 6H 2 = 7(NH 4 ) 2 S0 4 + 9 H 2 S0 4 + 2HN0 3 . 

The above equations indicate that the general effect of ammo- 
nium persulphate as an etching medium is that of a dilute acid, 
the action of which is intensified by the presence of free oxygen. 
In a previous article 3 the importance of available oxygen in 
metallographic etching reagents has been pointed out. It may 
be easily shown also that aqueous solutions of ammonium sul- 
phate have a considerable etching action upon segregated steels, 
hence it is very probable that the sulphate which is formed as one 
of the reaction products aids somewhat in the etching action of 
the persulphate solution. 


On account of the readiness with which ammonium persulphate 
crystals decompose when water is added to them, the etching solu- 
tion must always be freshly prepared just before use. One gram 
of the salt in io cm 3 of water was found to give excellent results. 
If desired the amount of persulphate may be increased considerably, 
for instance, to 2 g in io cm 3 of water. The salt dissolves grad- 
ually in the water, hence the exact concentration of the etching 
solution can not be stated. 

The etching is best carried out by means of a cotton swab which 
has been soaked in the solution. The swab is rubbed over the 
face of the specimen, considerable pressure being used. A fine 
polish of the specimen is unnecessary; usually it is sufficient to 
finish the face on fine emery paper — for instance, 00 French 
emery paper. The dark oxide film which often forms on the 
freshly etched face of the specimen if it is allowed to stand in the 
air without drying for a few minutes can be easily removed by a 
fresh application of the etching solution on the cotton swab. An 
etching period of from i to 2 minutes is usually sufficient. 

2 Treadwell, Analytical Chemistry, p. 595. 

3 Henry S. Rawdon and Marjorie G. Lorentz, B. S. Sci. Papers, No. 399. 

Rawdon] Macroetching with Ammonium Persulphate 717 

One of the advantages of the method of etching described is the 
readiness of control of the process. The surface is constantly 
under observation and the etching can be stopped at any instant 
when the structure is distinctly revealed, by holding the specimen 
in a running stream of water. 

The photographing of the etched surface of the specimen is best 
carried out while the specimen is immersed in water, alcohol, or 
light oil. In case the specimen is too large for immersion, the 
surface may be rubbed with oil or glycerin in order to increase the 
brilliancy of the etch pattern. In general, this device is helpful in 
the photographing of all macroscopic specimens. 


In the following figures a few illustrations, chosen from a con- 
siderable number of specimens etched by this method, are given to 
show the results obtained by the use of the reagent. They also 
illustrate the fact that in some cases features are revealed by the 
persulphate which would be missed entirely if sole dependence was 
placed upon some other reagent. 


Ammonium persulphate is most admirable for revealing crys- 
talline heterogeneity of any material. This is illustrated by Fig. 1 , 
which shows sections of two fusion welds made by the oxyacety- 
lene process. Etching with an aqueous solution of copper am- 
monium chloride and with alcoholic nitric acid both failed to reveal 
clearly the pronounced grain growth which accompanied the 
welding process and which influences to a very marked degree the 
mechanical properties of the weld. This is very clearly revealed 
by the use of the persulphate. . A single type of etching reagent 
should not be depended upon for completely revealing the macro- 
structure of any specimen. Each reagent used, as illustrated in 
Fig. 1 , reveals certain characteristic features of the material under 
observation. It appears, however, that the persulphate solution 
revealed more features in a single etching than did either of the 
other two in this particular case. 

In Fig. 2 a specimen of commercial wrought iron is shown and 
the results obtained by the use of ammonium persulphate may be 
compared with those of four other reagents in common use. 
Although the characteristic "streaky" condition is revealed by 
several, the peculiar variations in the crystalline condition are 

718 Scientific Papers of the Bureau of Standards Woi.i6 

Fig. i. — Crystalline heterogeneity of fusion welds of steel as 
revealed by different etching reagents. X I 

The same two welds were used for each of the three etching reagents. 
Etching reagents: a and b. Copper ammonium chloride (Heyn's reagent); 
candd, 2 per cent alcoholic nitric acid; e and /, aqueous solution of 
ammonium persulphate 

Rawdon) Macroetching with Ammonium Persulphate 


revealed only by the persulphate. In Fig. 3 the same specimen 
is shown at a slightly higher magnification in order to reveal the 
crystalline pattern more plainly. The material, when treated 
with the persulphate, has the appearance of being deeply etched. 

a b c d e 

Fig. 2. — Structure of "streaky" commercial wrought iron revealed by different etching 

reagents. X I 
The same specimen was used throughout. Etching reagents: a, 2 per cent alcoholic nitric acid ; b, aqueous 
solution of a m monium persulphate; c, copper ammonium chloride (Heyn's reagent); d, acidified solution 
of copper chloride (Stead's reagent); e, hot concentrated hydrochloric acid, 5 minutes immersion 

This is not so, however, as the etch pattern can be readily removed 
by rubbing the surface with emery flour. Fig. 2 represents the 
order in which the successive etchings of the specimen were carried 
out. In each case the previous etch pattern was removed by 

Fig. 3. — Crystalline structure of wrought iron revealed by etching with 

solution of ammonium persulphate. X J 
The same specimen shown in Fig. 2 is here shown at a slightly higher magnification 

rubbing the surface with emery flour on the tip of the finger. It 
may be noted in passing that this is an excellent method for freeing 
the surface of a specimen from finger prints and other traces of oil 
or grease before etching the material. 

720 Scientific Papers of the Bureau of Standards [Voi.16 


One of the most useful purposes served by macroscopic etching 
is the revealing of chemical unhomogeneity ; that is, variations in 

Fig. 4. — Chemical unhomogeneity in a bar of segre- 
gated steel as revealed by different methods of etch- 
ing. X 1 

Etching reagents: a, aqueous solution of ammonium per- 
sulphate; b, hot concentrated hydrochloric acid 

Fig. 5. — Chemical unhomogeneity in the head of a rail as revealed by different methods of 

etching. X 5J7 

Methods of etching: a, Sulphur print; b, copper ammonium chloride (Heyn's reagent); c, aqueous solu- 
tion of ammonium persulphate; d, deeply etched in hot concentrated hydrochloric acid, inked with 
printer's ink, and a print made of the etched surface 

chemical composition due to segregation, cementation, and other 
causes. Figs. 4 and 5 illustrate the fact that etching with persul- 
phate is usually sufficient to reveal such differences in composition. 

Rawdon) Macroetching with Ammonium Persulphate 721 

The chemical analysis of segregated specimen shown in Fig. 4 
indicated that the central and outer portions of the bar differed 
principally in the sulphur and phosphorus content. The per- 
centages were as follows: Phosphorus, outside 0.076 per cent, 
center 0.138 per cent; sulphur, outside 0.029 per cent, center 
0.064 P er cent. The appearance of the surface after deep etching 
in hot concentrated hydrochloric acid and that after etching with 
ammonium persulphate are quite similar. 

Fig. 5 shows a section of the head of a segregated rail which 
failed in service. The use of several of the common etching 
reagents is illustrated. The zone of segregation, which is largely 

Fio. 6. — Differences in composition of steel rivet 
as compared with the nickel-steel riveted plates, 
revealed by etching with ammonium persulphate 
solution. X 1 

due to sulphur as shown by Fig. 5a, is quite definitely indicated 
by the persulphate etching. 

Fig. 6 shows a section through a low carbon steel rivet which 
holds together two plates of 3^ per cent nickel steel. Only by 
etching with ammonium persulphate was it found possible to 
reveal any of the "flow lines" within the rivet. These are 
faintly shown in the figure. Other macroscopic etching methods 
showed only a difference in shade of the rivet as compared with 
the plates, indicating thus that the two differed very considerably 
in composition. 

Variations in structural condition due to local heating of parts 
of a steel specimen which may arise in such processes as welding, 
oxy acetylene cutting, etc., are also readily revealed by per- 
sulphate etching. Fig. 7 shows sections of the head of a steel 
rail to which a copper bonding cable has been welded at the side. 
The depth to which the metal has been affected by the heating 


Scientific Papers of the Bureau of Standards 

[Vol. 16 

is rendered plainly visible by etching with ammonium persulphate 
or with very dilute nitric acid. 


It is evident from the foregoing discussion and results that 
ammonium persulphate is a very useful reagent for revealing the 


Fig. 7. — Structural changes caused by heat treatment incidental to 
welding operations as rexealed by different etching reagents 
X 1 

The cross section of the rail head reveals the effect of the heating caused by 
welding on the copper bond at the side, a and b represent the same rail but 
different cross sections. Etching reagents: a, Aqueous solution of ammonium 
persulphate; b, 2 per cent alcoholic nitric acid 

macrostructure of iron and steel. Its action depends primarily 
upon the formation of acid and the liberation of oxygen from the 
spontaneous decomposition of the salt upon the addition of water. 
It is to be recommended particularly for revealing the crystalline 
condition of iron and steel although chemical unhomogeneity is 
also indicated by its use. 

Rawdon) Mocroetchiug with Ammonium Persulphate 723 

It is not to be inferred that the persulphate reagent is the only 
means by which crystalline heterogeneity may be revealed. 
Other methods, such as deep etching with concentrated acid, or by 
prolonged etching in dilute solutions — for instance, picric acid — 
will often reveal the same features. Such processes, however } 
are very tedious and time consuming and the face of the specimen 
becomes so roughened and pitted as to render it of no further 
value for examination without deeply grinding and repolishing 
the surface. In this way features close to the surface might be 
entirely removed between successive etchings. 

The cleanliness of the persulphate reagent and the ease with 
which it may be applied even to surfaces of considerable size are 
also important factors favoring the use of this method of etching. 

Washington, August 27, 1920.